Fluffy iron powder and process for preparing same



Nov. 21, 1967 'MSHAFER 3,353,951

FLUFFY IRON POWDER AND PROCESS FOR PREPARINQSAME Original Filed 001;. 1,1964 2 Sheets-Sheet 1 NOV. 21, w H F ER ETAL- FLUFFY IRON POWDER ANDPROCESS FOR PREPARING SAME Original Filed Oct. 1, 1964 2 Sheets-Sheet 2Fig.3.

MAGNIFICATION 50X -100 +150 MESH United States Patent 3,353,951 FLUFFYIRON POWDER AND PROCESS FOR PREPARING SAME William M. Shafer, CrownPoint, Ind., and George Yurasko, .lr., Syosset, N.Y., assignors to TheGlidden Company, Cleveland, Ohio, a corporation of Ohio Continuation ofapplication Ser. No. 400,873, Oct. 1, 1964. This application May 9,1966, Ser. No. 554,625 8 Claims. (Cl. 75.5)

The present application is a continuation of co-pending United Statespatent applications, Ser. Numbers 400,873 and 217,402; application Ser.Number 400,873, now abandoned, is a continuation-in-part of saidapplication Ser. Number 217,402. The disclosures of both applicationsare hereby incorporated herein by reference.

This invention relates to a novel iron powder and to a process for itsproduction. The invention more particularly relates to fiufiy or porousiron powder having low apparent density and low oxygen and carboncontents. The invention is advantageous in that a powder falling withinits scope, when compacted and sintered, has surprisingly more than twicethe sintered tensile strength and more than two-and-one half times thesintered modulus of rupture of previously known iron powders ofcomparable green strength and apparent density thereby rendering thepowders useful in the manufacture of compressed sintered metal articles.

The term sintered tensile strength as used herein is intended to meanand to refer to the tensile strength of the compacted and sintered metalpowders, having blended therewith 1 percent of zinc stearate as alubricant, which has been compacted at a pressure of 30 tons per squareinch in a standard Metal Powder Industries Federation tensile bar dieand sintered for 30 minutes at 2050" F. in a non-oxidizing atmosphere.

The term sintered modulus of rupture as used herein is intended to meanand to refer to the transverse modulus of rupture of compacted andsintered metal powders, having blended therein 1 percent of zincstearate as a lubricant, which has been compacted at a pressure of 30tons per square inch in a standard Metal Powder Industries Federationtransverse bar die and sintered for 30 minutes at 2050 F. in anon-oxidizing atmosphere.

The sintered tensile strength and sintered modulus of rupture valuesherein set forth were obtained using the standard procedures describedin Metal Powder Industries Federation Standards 10-63 and 13-62 revisedand published in 1962 and 1963, respectively, by the Metal PowderIndustries Federation.

In the accompanying drawings, FIGURE 1 is a photograph of magnifiedparticles of the fluffy or porous iron powder of this invention whichhas an apparent density of 0.67 gram per milliliter. The iron powdershown has a particle size of l00+150 mesh. The magnification of thepowder particles is 60 diameters. FIGURE 2 illustrates some of the sameparticles at a magnification of 120 diameters. FIGURE 3 is a photographof particles of iron powder having an apparent density of 0.64 gram permilliliter which were prepared according to the procedure of ExampleXXXIII. The mesh is the same as that of the powder illustrated inFIGURES 1 and 2. The magnification of the particles in the photograph is50 diameters. In all three figures, the porosity of the powders can bedistinctly seen. These iron powders have a sintered tensile strengthabove about 20,000 p.s.i. when pressed at 30 t.s.i. which is more thantwice that of the sintered tensile strength of iron powder of comparablefresh green strength and low apparent density made by previously knownprocesses. The iron powders also have a sintered 3,353,951 Patented Nov.21, 1967 modulus of rupture above about 50,000 p.s.i. when pressed at 30t.s.i. which is about two-and-one-half times that of iron powdersobtained from previously known processes for making iron powder ofcomparable green strength and apparent density.

The iron powder of this invention is additionally advantageous inaffording a means for controlling the change in size of a sinteredcompact which occurs when it is sintered. Most iron powders heretoforeknown, when mixed with copper powders and thereafter compacted andsintered show an expansion in all dimensions throughout.

the compact. It has been found that the iron powder of this inventionwhen similarly processed shows small reductions, rather than expansions,in dimensions. The novel iron powder can be mixed with presently knowniron powders and/or with copper powders to provide mixtures which, whencompacted and sintered, exhibit little or no change in dimension orexhibit controlled expansion or contraction in the dimension, asdesired. The iron powder of this invention can also be blended withpreviously known iron powders, with or without copper powder, to producesintered compacts having improved strength as a result of the strengthimprovement conferred by the novel iron powder.

The invention provides an iron powder substantially of the appearanceshown in the figures, particularly FIG- URE 3. The iron powder has anapparent density below about 1.0 gram per milliliter, an oxygen contentbelow about 2.0 percent, and a carbon content below about 0.2 percent.As previously noted, the powder also has a sintered tensile strength(when pressed at 30 t.s.i. and sintered) above about 20,000 p.s.i. and asintered modulus of rupture above about 50,000 p.s.i. when pressed at 30t.s.i. and sintered under the same conditions.

The present invention also provides a process for producing the ironpowder which comprises the steps of:

(a) forming a uniformly blended powdered mixture comprising:

(1) finely divided iron oxide having an oxygen content between about 6and about 30 weight percent, basis the weight of the iron oxide, andcontaining less than about one weight percent of acid insolubles;

(2) finely divided, solid, carbonaceous material in an amount sufficientto provide an iron oxide oxygen to free carbon atomic ratio in said miX-ture of from about 1.5:1 to about 3:1; and

(3) from about 0 to about 3 weight percent, basis the weight of the ironoxide, of a finely divided metal carbonate selected from the groupconsisting of alkali and alkaline earth metal carbonates; (b) contactingsaid mixture in a reduction zone at a temperature between about 1550 F.and about 1850 F. with a flow of hydrogen-containing reduction gashaving a dew point below about 120 F. for a time sufficient to reducethe iron oxide in said mixture and to form a coalesced mass consistingessentially of porous iron having an oxygen content below about 2.0percent and a carbon content below about 0.2 percent and cooling saidmass to below about F. under non-oxidizing conditions. The coalescedmass can then be readily comminuted into iron powder having thehereinafter defined particle size desired.

Iron powders produced by a process falling within the scope abovedescribed are advantageous in that sintered compacts having a greatlyincreased sintered tensile strength and sintered modulus of rupture arereadily obtained. Sintered iron powder compacts having the high sinteredtensile strength and sintered modulus of rupture have been heretofore,to the best of our knowledge, un-

obtainable from powders of comparable green strength and low apparentdensity.

During reduction, the blended mixture swells up or puffs leaving voidspaces in the reduced charge thereby rendering the reduced chargereadily susceptible to comminution and permitting ready separation ofthe iron powder from any small residues of non-ferrous solid materialwhich may be present.

The uniformly blended powdered mixture employed (comprising finelydivided iron oxide, finely divided solid, carbonaceous material, andoptionally, a metal carbonate) is prepared by conventional methods suchas by grinding the respective ingredients, usually by mechanical means,such as a ball mill, to the particle size desired and blending theground materials in a mechanical mixer until a uniform blend isobtained.

The finely divided iron oxide employed usually contains about 6.0 toabout 30 percent (preferably 19-30) by weight of oxygen and has acidinsolubles content below 1 percent. The finely divided oxide may be millscale, beneficiated high grade iron ore, magnetite or hematite, mixturesof the foregoing and the like. It is brought into finely divided orcomminuted form by grinding to particles of finer than about 30 mesh,preferably finer than about 80 mesh.

The iron oxide should contain less than 1 percent by weight of acidinsoluble material. If it contains more than this quantity, iron powderhaving the properties above defined will sometimes not be obtained dueto the difficulty in separating the insolubles from the finishedproduct.

The solid carbonaceous material (e.g., the reducing agent) can consistof any of a wide variety of free carboncontaining materials such as, forexample, charcoal, petroleum coke, lamp black and the like, which arefinely divided or are made finely divided by conventional grindingmethods. The solid carbonaceous materials are usually ground andcomminuted to a mixture whose particles are finer than 100 mesh andpreferably are finer than 325 mesh.

The amount of carbonaceous material in the uniformly blended mixturewill depend upon the amount of iron oxide and the amount of oxygenpresent therein. The amount should be that sufficient to provide an ironoxide oxygen to free carbon atomic ratio in the range of from about1.5:1 to about 3:1. If the oxygen to carbon atomic ratio is less thanabout 1.5:1, the reduced powder will often contain large quantities ofcarbon and some of the properties of flufl'iness, porosity of thepowder, and sintered tensile strength will be lost. If the oxygen:carbonatomic ratio is greater than about 3:1, the reduced iron powder productwill often contain undesirably large quantities of oxygen (iron oxide)and will lose a certain amount of its sintered modulus of ruptureproperties. In a preferred embodiment of a uniformly blended mixture,the iron oxide employed will contain between about 19 to about 30 weightpercent of oxygen and the oxygenzcarbon atomic ratio will be in therange of from about 2.25 :1 to about 3: 1. Differently stated, the solidcarbonaceous material used in preparing the uniform blend is soproportioned to the oxygen of the iron oxide that all or substantiallyall of the carbonaceous material will be consumed during the course ofthe reduction. It is therefore preferred to employ a percentage ofcarbonaceous material, calculated as 100 percent carbon, which isnumerically equal to about one-third the oxygen content of the ironoxide in the carbonaceous material with up to about 2 percent additionalcarbonaceous material, based on the iron oxide. By way of example, ifthe iron oxide contains say percent oxygen, then the solid carbonaceousmaterial is used in amounts which correspond to a range of 8.33l0.33weight percent, based on the weight of the iron oxide.

The amount of carbonaceous material will also depend to some extent onthe amount of reducing atmosphere employed. Thus, when the mixture isreduced in the form of a thin layer on a moving belt exposed to areducing atmosphere, less carbonaceous material will be employed than inthe case when the depth of the charge is greater, for example. when thecharge is placed in containers such as saggers, cans, or crucibles sothat the only reducing atmosphere at the bottom of the charge is thatgenerated by the mixture.

The finely divided metal carbonate (sometimes hereinafter referred to asan energizer) which can be employed in the mixture is an alkali metal oran alkaline earth metal carbonate. Examples of alkali metal carbonatesinclude sodium, lithium and potassium carbonate; examples of alkalineearth metal carbonates include barium, strontium and calcium carbonates.Of these metal carbonates, potassium carbonate is a preferred alkalimetal carbonate and barium carbonate is a preferred alkaline earth metalcarbonate since these carbonates have been found to provide iron powderhaving particularly desirable sintered compact properties.

The metal carbonate in the mixture can also be provided by metathesis,e.g.

or the metal carbonate may be provided as a product of thermaldecomposition, e.g.

The amount of metal carbonate can vary within the range of 0 to 3percent by weight depending upon the nature of the hydrogen-containingreduction gas and the particulate metal carbonate employed.

Where the hydrogen-containing reduction gas contains nitrogen such as,for example, in the ease of dissociated ammonia gas, the carbonate cansometimes be omitted or lowered. However, where the reduction gasconsists of substantially pure hydrogen gas, the metal carbonate isemployed at the higher percentages (e.g., up to 3 weight percent of theiron oxide). Where the metal carbonate is an alkali metal carbonate suchas sodium or potassium carbonate, a larger weight percentage of metalcarbonate has often been found desirable. Where the carbonate employedis an alkaline earth metal carbonate, such as, for example bariumcarbonate, from about 0.5 to about 1.5 weight percent of bariumcarbonate is employed in the uniformly blended mixture.

Although higher quantities of metal carbonate may be employed in theblended mixture, there is usually no advantage and difficulties maysometimes be encountered in separating the reduced iron from the metalcarbonate after reduction of the iron oxide has been achieved. Theparticle size of the particles of the finely divided metal carbonate inthe uniform blend is below mesh and preferably is substantially the samesize as the particles of the finely divided carbonaceous material inorder that a more homogenous blended mixture can be obtained.

As aforenoted, the novel iron powder of this invention is obtained bycontacting a uniformly blended mixture (above identified) in a reductionzone at a temperature between about 1550 F. and about 1850 F. with aflow of hydrogen-containing reduction gas having a dew point below aboutF. for a time sufiicient to reduce the iron oxide in said mixture and toform a coalesced mass consisting essentially of porous iron having anoxygen content below about 2 percent and a carbon content below 0.2percent. The reduction zone, in which contact of the mixture with thereduction gas is effected, can be any of a number of conventionalreduction zones known in the art such as, for example, reductionfurnaces in which are placed heat resistant metal cans such as thosedescribed in US. Patent 2,927,015, which contain the mixture or intocrucibles. Also, the mixture can be spread simultaneously from thereduced charge at about the same rate.

When the hydrogen-containing decarburization gas is employed, it isbelieved that the atmosphere around the charge, at least during theinitial stages of reduction, is not decarburizing (albeit the gasintroduced is of a decarburizing character) due to the presence of thecarbonaceous material in the mix. The dew point of thehydrogen-containing reduction gas (e.g., the presence of small amountsof moisture) is also of some assistance in decarburization.

The following specific examples are intended to illustrate theinvention, but not to limit the scope thereof, parts and percentagesbeing by weight unless otherwise specified.

Example 1 8.33 pounds of charcoal and 0.50 pound of barium carbonate arecharged to a ball mill and ground together to 100 mesh. The groundmixture is then blended with 100 pounds of 80 mesh mill scale containingoxygen by Weight. The resulting ternary mixture is spread in a /2 inchlayer on a moving belt which conveys it into a reduction furnace. Therethe mixture is heated for two hours at 1700 F. while maintained within areducing gaseous atmosphere composed of dissociated ammonia (e.g., 2NH N+3H After the two-hour period, the reduced charge is kept protected withthe dissociated ammonia atmosphere until it has cooled to about roomtemperature. It thereupon emerges from the furnace, is recovered fromthe moving belt and is passed through a magnetic separation unit whichseparates the iron powder effectively from unconsumed non-ferrous solidmatter.

The iron powder so recovered exhibits the following typical range ofqualities (after being ground 80 mesh):

Total carbon (percent) .02.06 Oxygen content (percent) .73.88 Apparentdensity g./ml .47.86 Pressed density 2 g./ml 6.10-6.25 Green strength 2p.s.i 920012,500 Sintered tensile strength p.s.i 21,00021,800

1 Weight.

2 Pressed at tons per square inch with 1.0% added zinc steairate aslubricant; standard M.P.I.F. (transverse) bar die use Pressed at 30 tonsper square inch with 1.0% added zinc stearate as lubricant in standardM.P.I.F'. tensile bar die and sintered for 30 minutes at 2050 F. in ahydrogen atmosphere.

Portions of the so-recovered iron powder are mixed with 7.5 Weightpercent of copper powder and 1 weight percent of zinc stearate, and theresulting mixtures are briquetted in a standard M.P.I.F. tensile barmold at 30 tons per square inch. The briquettes are then sintered for 30minutes at 2050 F. in a hydrogen atmosphere. The resulting bars aretested for strength and measured for dimensional changes. The averageresults are as follows:

Tensile strength p.s.i. 50,800 Dimensional change (shrinkage) percent.04

As noted hereinabove, present commercial iron powders, when similarlymixed, pressed, and sintered exhibit a growth in dimensions; e.g., from+0.44% to +2.80%.

Example 11 The mill scale of Example I (but ground to 100 mesh), thecharcoal and the barium carbonate of Example I are treated as theredescribed except for the variations noted below, said variations beingmade to illustrate the effect of barium carbonate and/or charcoal onsecuring reduction of the mill scale in two hours at 1700 F. under theconditions used in Example I.

Percentage reduction after Composition of charge: 2 hours at 1700 F.

(1) Ground mill scale only 40 (2) Ground mill scale plus 8.3 wt. percentground charcoal 74 (3) Same as 2 plus .25 wt. percent barium carbonate84 (4) Same as 2 plus .50 wt. percent barium carbonate 94 (5) Same as 2plus 1.0 wt. percent barium carbonate 97.5

Examples Ill-VIII In these examples, very fine iron oxide is used,namely iron oxide which has been precipitated chemically from an aqueoussolution of iron salt such as the ferrous sulfate described in US.Patents 2,560,970 and 2,560,971. Charcoal equal in weight to one-thirdof the hydrogen loss of the iron oxide is mixed and ground with 0.50%(Wt.) of barium carbonate. The charcoal/ carbonate mixture is then mixeduniformly with the precipitated iron oxide. Portions of the resultingmix are then reduced in dissociated ammonia at the temperatures and forthe time periods shown below in Table I. After the reduced charge hasbeen cooled in the dissociated ammonia atmosphere, it is ground to 200mesh powder, magnetically separated from nonferrous solid matter, andanalyzed. Table I also shows the apparent densities, hydrogen losses andtotal carbon contents of the powders secured from the respectivestarting portions of the ternary reduction mixture.

TABLE I Reduction Minus 200 Mesh Powder Example Temp, Time ApparentHydrogen Carbon F. (Min.) Density Loss (percent) (g./ml.) (percent) III1, 700 120 84 to 1. 01 18 01 1V 1, 600 120 77 28 07 Examples IX-XIIIMill scale is here roasted and then ground to 100 mesh, the screenanalysis being as follows.

Mesh: Percent On 4.00

The resulting ground iron oxide has a hydrogen loss of 26.27% (wt.), atotal carbon content of 0.73% (wt.) and an acid insolubles content of0.30% (wt.).

evenly on a thin (M4 to A2 inch thick) layer on a moving belt. Themixture may even be pelletized prior to reduction. Whatever method isused in carrying the uniformly blended mixture into the reduction zoneto contact the mixture with the flow of reduction gas, the aim is toexpose the mixture blend in a furnace at a controlled temperature andkeep it in contact with a flow of hydrogencontaining atmosphere untilthe iron oxide component of the mixture has been reduced to metalliciron containing less than 2 percent, preferably less than 1 percent ofoxygen, based on the weight of reduced iron analyzed. A conventionalmethod of determining the oxygen content of the reduced iron is tomeasure the loss in weight of the product by treating it with dry oxygenfree hydrogen at 1300 C. This loss in weight is referred to in the artas hydrogen loss to identify the oxygen content of hydrogen reducibleoxides in metal powder products.

During the reduction, the mixture is heated to and maintained attemperatures between about 1550" F. and 1850 F., preferably between 1650F. and 1750 F. While the mixture is at these temperatures, it is exposedto the reducing action of a hot stream of hydrogen-containing reductiongas at a slow to moderate rate such as, for example, between about 2 toabout cubic feet per hour per pound of oxidic iron. After a period oftime, usually pre-determined by several experiments conducted in theparticular reduction zone or furnace to determine how long the mixturemust be contacted to effect complete reduction of the iron oxide, thereduced mixture is cooled to below about 100 F., preferably to aboutroom temperature, while protecting it from re-oxidation by means of thegaseous reducing atmosphere employed in the reduction or by any othersuitable non-oxidizing atmosphere.

The reduced mixture so cooled can then be separated from unconsumedsolid carbonaceous material and from residual carbonates by anyappropriate means such as by passing it over a magnetic pulley, bygently blowing it while passing it over stationary magnets or magneticpulleys, or by merely winnowing.

In one embodiment of a process of this invention, the finely dividediron oxide is first partially reduced by gas eous and/or solid carbonreduction wherein the metal carbonate is omitted from the uniformmixture to a stage wherein the oxygen content of the starting materialhas been lowered to about 6 to 10 percent by weight. The product is thenstill sufiiciently magnetic to be magnetically separated from unconsumedcarbon and is also sufiiciently friable to permit easy grinding to thefinely divided state. The partially reduced iron oxide is then employedas the starting iron oxide in the process and mixed with additionalcarbonaceous material and metal carbonate when the latter is used.

The hydrogen-containing reduction gas employed to contact the uniformmixture in the processes of this invention can be any hydrogen rich gasincluding pure hydrogen, dissociated ammonia,steam-reformed-natural-gas, water gas, endothermic gas prepared byheating a mixture of water vapor or steam and methane, mixtures thereof,or other numerous similar reducing gases having a hydrogen content of atleast about percent by volume. Such gases may contain moisture, butshould have a dew point below about 120 F. at the time of introductioninto the zone. If gases containing moisture such that they have a dewpoint above about 120 F. are employed, the reduced iron product willsometimes have an undesirably high oxygen content. As will behereinafter evident, where a metal carbonate is not employed, thereduction gas should contain from about to about percent by volume ofnitrogen gas. A specific advantageous example of a nitrogen and hydrogencontaining gas which may be employed in the reduction of mixtures whichdo not contain metal carbonate is dissociated ammonia.

As previously noted, the temperature in the reduction zone and that ofthe mixture when charged is a temperature between 1550 F. and 1850 F. Iftemperatures below about 1550 F. are employed, reduction times willoften be unduly prolonged. Although temperatures above 1850 F. can beemployed, there is usually no advantage and the process becomesuneconomical. From the foregoing description and as will be hereinafterevident from the specific examples, the process of this invention notonly involves critical amounts of carbonates or nitrogencontainingreduction gas, but also necessitates:

(a) Careful control of the amount of solid carbonaceous material in theuniformly blended mixture; and

(b) Reduction of iron oxide in the presence of a hydrogen-containingatmosphere.

The reasons why these and certain other factors cooperate to produceiron powder having the properties hereinbefore defined are not knownwith certainty. However, it is believed that the properties are due tothe swelling up or puffing of the charge during reduction and that thepulling up of the charge results from diverse catalytic actionsoccurring in sequential, but partially overlapping order.

Although applicants have no wish or intent to be bound by theory, thefollowing circumstances are believed to co-act in some way to producethe novel iron powder of this invention.

By the use of solid carbon in nearly stoichiometric amounts, based onthe oxygen content of the iron oxide, substantially all of the ironoxide is reduced when most of the carbon has been consumed therebyinsuring a relatively high content of metallic iron while there is stillenough carbon remaining to maintain carburizing conditions in thecharge. The maintenance of carburizing conditions during the time whenhigh quantities of metallic iron are present results in carburization ofthe iron due to known catalytic effects of the metal carbonates and/ornitrogen. Carbon such as graphite deposited on saturated carburized ironcauses reduction of iron oxides, and the carbon of the carburized ironalso causes such reduction. By the time carburization of the reducediron has occurred, there are still significant quantities of unreducediron oxide in the charge and this oxide and the gaseous atmosphereconvert the reaction from one of carburization to one ofdecarburization. When decarburization prevails, the oxygen in the ironoxide is partially reduced, is in the form of ferrous, rather thanferric oxide, and is soon exhausted. Gaseous hydrogen and the moisturecontent of the flowing gas then produce and maintain a condition ofdecarburization. Carbonaceous materials low in sulfur are employed toavoid conversion of the metal carbonates to corresponding unreactedmetal sulfides. It is thus postulated that the invention involves thereduction of iron oxides which are first reduced in part to iron, thereduced iron is carburized, and the decarburized iron functions duringits decarburization to assist in the reduction of the remaining ironoxide.

The metal carbonates, when used, are brought to a comminuted state so asto be distributed uniformly throughout the blended mixture, The solidcarbon is also finely divided so as to improve the probability of havingsuflicient carbon close to metallic iron in order to insure effectivecarburization. When carbonates are not used, carburization is attainedby using a reducing atmosphere rich in both gaseous nitrogen and gaseoushydrogen (e.g., dissociated ammonia). In this instance, the nitrogenrather than the metal carbonate, serves as the carburization energizer.

The particle size of the iron oxide particles is not critical, but whencarbonates are not used, it is desirable to have the iron oxides in amore finely divided state (e.g., particles less than mesh and finer). Anexcess of solid carbon over stoichiometric proportions, based on theoxygen content of the starting iron oxide, is deliberately avoided sothat decarburization can begin near the end of the reduction process.This is accomplished by having both iron oxide and carbon disappearsubstantially TABLE II Batch Ground Charcoal, Carbonate percent None.

0.259 (Wt.) SI'CO3 0.50% (Wt.) SrCOa 1.00% (Wt.) SrCOa 0.50% (Wt) BaCOsEach blended binary or ternary mixture containing iron 10 oxide and oneof the batches of charcoal is placed in a metal pan to a depth of 0.50inch. Then the pans are placed side by side in a reduction furnacewherein they are all exposed to the same conditions; namely, reductionat 1720" F. for two hours in an atmosphere of dissociated ammoniaflowing through the reduction chamber at a rate of 70 cubic feet perhour, followed by cooling to room temperature in the same atmosphere.

The cakes of reduced charge are removed from the pans, fed through abrush grinder and screened through an 80 mesh screen. The 80 meshmaterial is analyzed for hydrogen loss, total carbon and apparentdensity. Table III summarizes the results and also includes the swelledor puffed height of each cake.

Examples XVIII-XX Percent Hydrogen 70.0 Carbon monoxide 20.6 Carbondioxide 3.5 Nitrogen 4.5 Methane 0.5

Dew point, +l20 F.

The ternary mixture using K CO is run in the samesteamreformed-natural-gas atmosphere. In each run, the ternary mixtureis spread on the moving belt to a depth of /2 Examples XIV-XVII Thereductions described in Examples X-XIII are repeated except that thecharcoal batches are formulated to include an added 2% of charcoal(total 10.76%, based on iron oxide) While the levels of the addedcarbonates are retained unchanged. Table IV summarizes the variationsand the results.

inch, and the layer is passed through the furnace at a rate such that itis heated to and kept at 1750 F. for two hours after which it is cooledin the same atmosphere to room temperature. The gaseous atmosphere flowsthrough the tunnel of the furnace at a rate of 200 cubic feet per hour.Table V summarizes the formulations and results.

TABE IV Minus Mesh Powder Example Charcoal Batch Hydrogen Total ApparentSwelled Loss (Per- Carbon Density Height,

cent) (Percent) (g./ml.) inches Ch plus p.25 SrCO3). 1.12 0.17 0.74 Chplus 0.50 SrCO 1. 52 0.11 0. 54 V-1 Ch plus 1.00 SrCO3). 1. 04 0. 04 0.55 %-1% Ch plus 0.50 BaCOa) 0.68 0.05 0.60

TABLE V Example XVIII XIX XX l Reducing gas Diss. NH; SRNG 1 i SRNG 1Percent Unroasted iron scale (wt.) 91.27 91. 27 91.27 Percent Charcoal(\vt.) 8. 33 8.33 1 8.33 Percent BaCO; (\vt.) 0.50 0.50 l Percent K2003(wt.) 0. 50 Percent Hydrogen loss of iron powde 0. 50 0. 60 0. 6t:Apparent density (g./rnl.) 0. 64 0.60 0. 55 Green modulus of rupture,p.s.i. 10, 500 10, 200 9, 620 Sintered tensile strength, p.s.i. 21, 80020, 900 22, 100

1 Steam-reformed-natural-gas. 2 Samples pressed at 30 tons per squareinch.

charge:

Percent Reduction due to reaction with hydrogen 40 Reduction due tosolid carbon 34 Reduction due to carbonate energizer and its reactionswith solid carbon 22 Comparable results are secured when thesteam-reformed-natural-gas atmosphere of Examples XVIII-XX is replacedwith endothermic gas having the following 12 Example XXV In thisexample, the following materials are used:

Roasted mill scale- Percent Oxygen content (hydrogen loss method) 26.27Carbon content 0.73 Acid-insolubles content 0.30 Screen analysis after3-hour ball milling (mesh).

Charcoal as solid reducing agent (Example A);

Petroleum coke as solid reducing agent (Example B);

Hydrogen (Recovered as by-product of an electrolytic process forproducing bleaching compound) as gaseous reducing agent; +55 F. dewpoint.

The reduction is carried out in a moving belt furnace having threesuccessive hot zones held at temperatures of 1618 F., 1650 F. and 1688F. respectively. Reduction time is 2.93 hours in the hot Zone of thefurnace,

obtained by using a belt speed of 7.5 ftrhr. The charges lbs. each)identified in the following table are distributed on the belt in layersabout /2 inch thick, in the sequence indicated in the table. The furnaceis, of course, thoroughly flushed initially with hydrogen and thereafterkept under a positive hydrogen pressure to repel inleakage of air. Apilot flame of burning hydrogen at the exit of the furnace is keptburning at all times to show existence of positive hydrogen pressure.After the charges pass beyond the hot zone. they are cooled to roomtemperature in the furnace atmosphere.

typlcal ifi i g, Y i gg i y ffi fib After reductlon, the cold reducedcharge is llghtly bon mQnoXl e Imogen me ane milled to reduce it tominus 100 mesh. The analysis for Examples XXI-XXIV total carbon,hydrogen loss (oxygen content) and ap- The following tables summarizetypical data representa- P f denslly are made 9 the milled p The tive ofiron powders reduced by the process described in 40 .mlned Powders are211.50 bnquetted at 30 p per Square Example I except for varying thebarium carbonate con- Inch of 21116 Stearate as and the tent between 050and 3 by weight pressed densities and green strengths are determinedfrom TABLE VI the briquettes by the usual standard methods. The

reported figures represent an average of two tests. BaCOa Total HydrogenAcid Apparent The charges are prepared in the following ways Ex(Percent) Carbon ILoss IIigSOlUlIItOS ?0nsilty C n- 1;

t (Percent) elem) men) 1 Elm Example A.Roasted scale plus charcoal equalto /1 of the oxygen content plus 0.5% barium carbonate all XXI 0.50 0.200.'0 l None 0.39 XXII 1.00 0.11 0.53 0. 23 0.95 mixed together andball-mllled for three hours.

38 8 3; 3%; 8:12 8:33 Example B.-Same as Example A except forreplacement of the charcoal with petroleum coke.

TABLE VIII Total i Oxygen I Apparent Pressed Green Sample l CarbonContent Density Density Strength I (Percent) 1 (Percent) (g./ml.)(g./!nl.) (p.s.i.)

Control J0 9.31 i l Example A" 02 1 0 73 I 0.47 o. 11 0.200 Example 13..1.09 l 0.25 0.77 6.24 9,400

TABLE VII When the partially reduced roasted mill scale (con- Pressed 1Green Modulus? b Radial? 6o trol of Table VIII is recycled by mixing itWllll about Example Density Strength of Rupture I Growth 3.10% of carbonand 0.5% sodium carbonate, it yields (g-lml.) (p- (P- (Percent) reducediron powder of zero carbon content having an I apparent density belowabout .9 gram per milliliter. 6.11 10, 250 107, 730 -0. 48 6.14 10,000111,300 2 -0.01 t XXVI 6.19 9.330 10,680 0 09 The followlng materialsare used: 6.17 8. 9.950 91.5% Mill scale; oxygen content (hydrogen loss)25.0%; 1 Mixed lwith 1% zinc stearate lubricant and pressed at 30 tonsper 5% Ground charcoal: 5 Hart! 11C q? Mixe d with 7% copper powder and1% zinc stearate, pressed at 30 tons (1.5% Barium carbonate. per squareInch and smtered [or .50 minutes at 2,050 I 1n hydrogen. 75 Theforegoing materials in the indicated p p 13 are mixed together andball-milled for about three hours. The ground mixture has the followingscreenanalysis.

Mesh:

Portions of the ground charge are placed in iron trays to a depth of 0.5inch and are reduced by placing the trays on a moving mesh belt whichconveys them through a tube furnace, the hot zone of which is kept at1730- 1750 F. The furnace contains an atmosphere of dry bottled hydrogenfor one pair of tests (Example A), and an atmosphere of moist hydrogenfor the other pair of tests (Example B). The moist hydrogen atmosphereis secured by bubbling bottled hydrogen through water. The trays aremoved through the furnace at a rate such that the charges in the traysare in the hot zone of the furnace for 1.75 hours, and are cooled toroom temperature in the furnace atmosphere.

After reduction, the cold reduced layers are broken up easily in thehands and are screened through an 80 mesh sieve. The 80 mesh powder isthen analyzed for particle size distribution, oxygen content, andapparent density. Portions are also briquetted at 30 tons per squareinch with 1% of zinc stearate as lubricant and the briquettes are testedfor pressed density and green strength by standard test methods. Thereported figures represent an average of two tests:

The following table summarizes the results:

TABLE IX Exmple A Fxample B (Dry (Moist Hydrogen) Hydrogen) Dew point ofatmosphere, F -58 56 Flow rate of atmosphere, c.f.h. 65 65 Screenanalysis of powder:

+100 mesh, percent. 1. 7 3. 8 +200 mesh, percenL 37.8 39. 9 +325 mesh, Lercemt. 30.1 26. 325 mesh, percent 30. 4 29. 8 Oxygen content of powder,percent 26 .31 Total carbon in powder, percent 01 01 Putfing observed?Yes Yes Apparent density of powder, g./ml .88 78 Pressed density ofbriquette, g./ml 6.21 6.19 Green strength of briquette, p.s. 9, 896 10,300 Sintered tensile strength, p.s.i 21, 800 000 l Cubic feet per hour.

2 Pressed at 30 tons per square inch with 1.0% added zine stearate aslubricant in standard M.P.I.F. tensile bar die and sintered for 30minutes at 2,050 F. in a hydrogen atmosphere.

Portions of the so-recovered iron powder are mixed with 7.5 weightpercent of copper powder and 1 weight percent of zinc stearate, and theresulting mixtures are briquetted in a standard M.P.I.F. tensile barmold at 30 tons per square inch. The briquettes are then sintered for 30minutes at 2050 F. in a hydrogen atmosphere. The resulting bars aretested for strength and measured for dimensional changes. The averageresults are as follows:

Sintered tensile strength p.s.i. 50,800 Dimensional change (shrinkage)percent .04

As noted hereinabove, present commercial iron powders, when similarlymixed, pressed, and sintered, exhibit a growth in dimensions; e.g., from+0.44% to +2.80%.

Example XX VII The roasted mill scale of Example XXV (but ground to l00mesh), the charcoal and the barium carbonate of Example XXV are treatedas there described except Percent for the variations noted below, saidvariations being made to illustrate the effect of barium carbonateand/or charcoal on securing reduction of the mill scale in two hours at1700 F. under the conditions used in Example XXV.

Percentage reduction Composition of charge: after 2 hrs. at 1700 F. (1)Ground mill scale plus 8.3 wt. percent ground charcoal 74 (2) Same as 1plus .25 wt. percent barium carbonate 84 (3) Same as 1 plus .50 wt.percent barium carbonate 94 (4) Same as 1 plus 1.0 wt. percent bariumcarbonate 97.5

Examples XXVlII-XXXII Mill scale is here roasted and then ground tomesh, the screen analysis being as follows: 1

Mesh: Percent On 4.00

The resulting ground iron oxide has a hydrogen loss of 26.27% (Wt.), atotal carbon content of 0.73% (Wt.) and an acid insolubles content of0.30% (Wt.).

Charcoal is ball-milled with or Without the indicated carbonate to afiness corresponding to a Fisher number less than three microns. Theground charcoal or charcoal/ carbonate mixtures are then mixed withportions of the ground iron oxide in amounts such that the charcoalamounts to 8.76% (Wt.) based on the weight of iron oxide (i.e.,one-third of the hydrogen loss of the latter). A portion of the groundiron oxide, without charcoal or carbonate, is used as a control. Table Xshows the various charcoal batches used in the subsequent reductionshereof.

TABLE X Ground Charcoal, Carbonate percent None.

0.25% (Wt.) SICOa. 0.50% (Wt.) SrCO 1.00% (Wt.) SrCO 0.50% (Wt.) BaCOa.

Each blended binary or ternary mixture containing iron oxide and one ofthe batches of charcoal, is placed in a metal pan to a depth of 0.50inch. Then the pans are placed side by side in a reduction furnacewherein they are all exposed to the same conditions; namely, reductionat 1720 F. for two hours in an atmosphere of dry bottle hydrogen flowingthrough the reduction chamber at a rate of 70 cubic feet per hour,followed by cooling to room temperature in the same atmosphere.

The cakes of reduced charge are removed from the pans, fed through abrush grinder and screened through an 80 mesh screen. The 80 meshmaterial is analyzed for hydrogen loss, total carbon and apparentdensity. Ta

ble XI summarizes the results and also includes the swelled height ofeach cake.

TABLE XI Minus 80 Mesh Powder Example i Charcoal Batch Hydrogen TotalApparent I swelled Loss 1 Carbon Density lleigiit, i (percent) (percent)1 t'gfinl.) inches xxvnr A (Charcoal onl v).. 96 0.29 g 0. 04 XXIX B((111 1 plus 0.25 SrCO-rL. 2.10 0.13 0.53 1 XXX. 1 C (Ch 1 plus .50S!C() .1 1.58 1 0.05 58 1 XXXIUH I) (Ch 1 plus 1.00 SrCOi). 1.12 i 0.00l). 67 l 1% XXXII E (Ch plus 0.50 BiLCUg) 1.88 0.00 i 1). 69 1 1%Control No charcoal and no carbonate .1 12. 44 1 0.00 .17

1 Charcoal.

Examples XXXIII-XXXV 13 Samples of each of the above were pressed attons Fifty pounds of mill scale were dried at 1000 F. for 30 minutes inan air oven to remove moisture. The dried mill scale was then ground for4 hours in a ball mill. The ground product had an oxygen content(hydrogen loss) of 25.89 percent and had the following screen analysis:

Screen mesh: Percent +20 -20 Trace 40 0.5 60 0.7 80 9.0 l00 18.0 l50+200 19.8 --200 +250 6.2 ---250 +325 12.7 -325 33.1

Three separate 378 gram portions of 100 mesh charcoal were respectivelyball milled with 22.7 gram portions of -100 mesh barium carbonate,calcium oxide and fine- 1y divided lamp black. Each mixture wasthereafter thoroughly blended with separate 10 pound portions of theabove described dried mill scale. The mixture containing bariumcarbonate was labeled A, that containing calcium oxide B, and themixture containing lamp black Was labeled C."

The mixtures were then heated to 600 F. and separately charged to atwenty foot long moving belt furnace. The samples were layered on thebelt. layers having a thickness of approximately 0.5 inch. The furnacewas maintained at 1700 F. A stream of hot (1700 F.) gaseous dissociatedammonia was charged through the furnace at a rate of 100 standard cubicfeet per hour. The belt moved the layered mixtures through the hot zonewhere they were held for 2 hours during which time the layers were incontact with the flowing gas stream.

The reduced mixtures then passed to a cooling zone at 2 inches perminute belt speed Where they were permitted to cool to room temperaturewhile being contacted with cooled dissociated ammonia which flowedthrough the cooling zone at 50 standard cubic feet per minute.

After cooling, the three coalesced masses were recovered. Each wasseparately ground so that the resultant particles passed through an 80mesh screen. The ground, reduced products resulting from mixtures A, B,and C had the following screen analysis:

p.s.i. in a standard M.P.I.F tensile bar die and sintered for 30 minutesat 2050 F. in an atmosphere of dissociated ammonia. Prior to sintering,the pressed density and green strength were determined. After sinteringthe sintered density, sintered tensile strength (in p.s.i.) and sinteredmodulus of rupture (in p.s i.) were measured using standard M.P.I.F.procedures. The results are summarized in Table XII.

TABLE XII.oENERAL, PRESSED AND SINTEREI) PROPERTIES A i B C Tot 11Carbon. percent g 0.15 0.10 l 0.10 Hydrogen Loss, percenL. 0.80 1. 02.l 1. 60 Apparent Density, percent..." 0.67 0.68 l 0.64 Pressed Densitytgms./ec.). 5.94 5. 87 i 0.86 Green Strength (p.s.i.) i 9.090 8.600 i8.990 Sintt red Density (gins lcc) l 6.03 5.91 i 5.90 sintered TensileStrength (p.s.i.) 24, 000 21, 000 21,800 stint-red .\1odulus of Rupture(p.s.i.) 57,300 50,100 town As is evident from the foregoing table, thesintered tensile strength in each instance was above 20,000 p.s.i. andthe sintered modulus of rupture in each instance was above 50,000 p.s.i.

The experiment of the preceding examples was repeated using 8.15 poundsof mill scale, 1.8 pounds of charcoal, and .05 pound of bariumcarbonate. The material was passed through the reduction zone (withoutcontacting it with hydrogen-containing gas) at 1800 F. at the same bedthickness and for the same time as in the preceding examples. Thesintered tensile strength of this material pressed at 30 t.s.i was 7800p.s.i. and the sintered modulus of rupture of the compact pressed at 30t.s.i. was 21,000 psi. In these latter instances, compacting andsintering procedures were identical to those described in ExamplesXXXIII-XXXV. It is evident that by employing the process of thisinvention, which involves a combination of an external flow of reducinggas and solid carbonaceous material, the sintered tensile strength wasbetween 2 and 3 times that of the conventional method wherein areduction atmosphere is produced in situ solely by the reacting ofcarbon with the iron oxide oxygen. Also, the sintered modulus of rupturewas about two-and-a-half times greater for the sintered compactsproduced from the iron powders of this invention than iron powdersproduced by previously known methods hereinbefore referred to.

What is claimed is:

1. A process for producing iron powder which comprises the steps of:

ta) forming a uniformly blended powdered mixture comprising:

(1) finely divided iron oxide having an oxygen content between about 6and 30 weight percent, basis the weight of the iron oxide, andcontaining less than about one weight percent of acid insolubles;

(2) finely divided, solid, carbonaceous material, in an amountsufficient to provide an iron oxide oxygen to free carbon atomic ratioin said mixture of from about 1.5:1 to about 3: 1; and

(3) optionally up to about 3 weight percent, basis the weight of theiron oxide, of a finely divided metal carbonate selected from the groupconsisting of alkali and alkaline earth metal carbonates;

(b) contacting said mixture in a reduction zone, at a temperaturebetween about 1550 F. and about 1850 F., with a flow ofhydrogen-containing reduction gas having a dew point below about 120 F,for a time sufiicient to reduce the iron oxide in said mixture and toform a coalesced mass consisting essentially of porous iron having anoxygen content below about 2.0 percent and a carbon content below about0.2 percent; and

(c) cooling said mass to below about 100 F. under non-oxidizingconditions.

2. The process of claim 1 wherein said mixture is contacted with a flowof said hydrogen-containing reduction gas at least about 2 standardcubic feet per pound of oxidic iron fed to said reduction zone.

3. The process of claim 1 wherein said coalesced mass containing saidporous iron having said oxygen and said carbon content is comminutedinto iron powder.

4. The process of claim 1 wherein the finely divided iron oxide containsbetween about 19 and 30 Weight percent of oxygen, the carbonaceousmaterial is selected from the group consisting of charcoal and petroleumcoke and is present in an amount suflicient to provide an iron oxide 18oxygen to free carbon atomic ratio of between about 2.25:1 and about3:1.

5. The process of claim 4 wherein the temperature in the reduction zoneis between about 1650 F. and 1750 F. and the reduction gas isdissociated ammonia.

6. The process of claim 4 wherein the uniformly blended powdered mixturecontains from about 0.1 to about 3.0 percent by weight of an alkalineearth metal carbonate and the reduction gas is hydrogen.

7. The process of claim 6 wherein the alkaline earth metal carbonate isbarium carbonate.

8. Iron powder having an apparent density below about 1.0 gram permilliliter, an oxygen content below about 2.0 percent and a carboncontent below about 0.2 percent, said powder having a sintered tensilestrength above about 20,000 psi. when pressed at t.s.i. and a sinteredmodulus of rupture above about 50,000 psi. when pressed at 30 t.s.i.

References Cited UNITED STATES PATENTS 2,947,620 8/ 1960 Whitehouse etal. -.55 3,069,158 12/1962 Wulfi 75--.55 3,126,276 3/ 1964 Marshall atal 75-26 DAVID L. RECK, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

1. A PROCESS FOR PRODUCING IRON POWDER WHICH COMPRISES THE STEPS OF: (A)FORMING A UNIFORMLY BLENDED POWDERED MIXTURE COMPRISING: (1) FINELYDIVIDED IRON OXIDE HAVING AN OXYGEN CONTENT BETWEEN ABOUT 6 AND 30WEIGHT PERCENT, BASIS THE WEIGHT OF THE IRON OXIDE, AND CONTAINING LESSTHAN ABOUT ONE WEIGHT PERCENT OF ACID INSOLUBLES; (2) FINELY DIVIDED,SOLID, CARVONACEOUS MATERIAL, IN AN AMOUNT SUFFICIENT TO PROVIDE AN IRONOXIDE OXYGEN TO FREE CARBON ATOMIC RATIO IN SAID MIXTURE OF FROM ABOUT1.5:1 TO ABOUT 3:1; AND (3) OPTIONALLY UP TO ABOUT 3 WEIGHT PERCENT,BASIS THE WEIGHT OF THE IRON OXIDE, OF A FINELY DIVIDED METAL CARBONATESELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH METALCARBONATES; (B) CONTACTING SAID MIXTURE IN A REDUCTION ZONE, AT ATEMPERATURE BETWEEN ABOUT 1550*F. AND ABOUT 1850*F., WITH A FLOW OFHYDROGEN-CONTAINING REDUCTION GAS HAVING A DEW POINT BELOW ABOUT 120*F.FOR A TIME SUFFICIENT TO REDUCE THE IRON OXIDE IN SAID MIX-